According to the prior art such fasteners are conventional by manufactured by upsetting and extrusion of a length of wire in several steps. This technology requires a large number of tools and dies, and transfer of the workpiece from one step to the next requires time. Furthermore, the tools are subjected locally to very high stresses, which may detrimentally affect their service life. In addition, radial undercuts such as outer grooves and knurls or inner threads require additional operations, in many cases even costly machining. Moreover, the conventional technology has many limitations, particularly in the case of relatively deep cavities of small diameter or cross-section, especially cavities of conical shape with a small included angle, since the cavity has to be formed by a die-sided pin. Because of a self-locking effect caused by friction, it is difficult to remove the finished parts from the pin, and pins of substantial length generally lack strength and stability required for the production process. Experience has shown that cavities made in a multi-step extrusion process often have small cracks at their bottom and that the fastener shank will not have at its free end a "sharp edge" or a well-defined flat due to limitations of the presently used production technique. This results in a non-optimal setting behaviour and thus in poor joint properties both in terms of appearance and strength. Finally, due to the limitations posed by the present production techniques it is not possible to economically manufacture such fasteners of materials such as austinitic stainless steel by cold-forming.
It is an object of the present invention to provide a method of manufacturing fasteners which does not suffer from the above-mentioned disadvantages and limitations. Such method should allow to make fasteners and similar parts which, as regards depth and geometry of the central cavity, do not have the above-mentioned limitations. In addition, the production process is to be simplified, the output per unit time is to be increased, and both the economics of the method as well as the quality of the products are to be improved. Furthermore, the method of the invention should allow to economically produce fasteners of materials which are difficult to process by using conventional upsetting and extrusion methods.
To this end, the present invention provides a method of manufacturing a fastener of a predetermined geometry and having a central axis and a central cavity, in which method a blank is provided and said blank is subjected to a rolling process causing material of the blank to flow radially towards said central axis and axially so as to axially lengthen the blank, with said predetermined geometry of said finished fastener being formed by said rolling process.
In accordance with the invention the blank is generally made from a sheared-off length of wire. The end faces of the blank may be planar; alternatively one end face thereof may be provided with a recess obtained by upsetting and/or extrusion.
Subsequently, the blank is formed into the finished fastener by a rolling process. Such rolling process causes the material of the blank to flow radially towards the center of the blank and axially such that the length of the preformed blank is increased and the required geometry of the finished fastener is obtained. The result is a fastener with a cavity of a desired geometry even when starting from a blank having planar end faces.
If, on the other hand, the blank is preformed so as to have a recess at one of its end faces, the rolling process causes material of the blank to flow from an area radially outwards of the cavity radially inwards and in an axial direction until the fastener will obtain its final shape.
The rolling process is a single stage operation, i.e. the rolling dies form the finished fastener out of the blank in a single step. A pre-requisit for this is that the profile of the rolling dies changes continuously--along the length of the rolling dies--from a profile corresponding to that of the blank up to that of the finished fastener.
Various types of rolling tools are suitable for performing the rolling process, namely such having flat rolling dies which are linearity movable with respect to each other, rotary cylindrical rolling dies, and rolling dies of cylindrical segments. In each case the tools are provided at their opposite outer surfaces with a continuously changing profile. When a pair of cylindrical rollers or rollers segments are employed, a stationary straight edge is used to support and guide the blank during rolling. In another version three rollers are used for providing the fasteners with a predetermined external profile by embossing. Such rolling tools are known in the art and have been conventionally used e.g. for roll-forming of threads on rods, screws and bolts. A major difference between the prior art and the invention is that in case of the latter the rolling process is employed for producing hollow or semi-hollow parts of defined outer and inner geometries.
The method of the invention can be practiced with or without a mandrel. As has been mentioned already, a central cavity of the fastener may be obtained simply by rolling. If, however, a precisely defined geometry of the central cavity is to be attained, it is preferred to use a rotating mandrel which is axially spring-loaded and which is urged into a recess of the blank. Since flow of material is radial from the outside to the inside, there is no self-locking tendency; due to the mandrel being spring-loaded, there will be a gap between the finished fastener and the mandrel, which facilitates removal of the finished fastener from the mandrel.
On the other hand, the method of the present invention allows to make fasteners having through-holes. In this case it is advisable to use a blank provided with a hole. Again the rolling process causes material to flow in both radial and axial directions until the desired geometry will be obtained. It is advisable to use a mandrel for producing a cylindrical bore. The mandrel may be provided with outer threads so that the fastener will be provided with internal threads by the rolling operation. The outside of the fastener may be of cylindrical or polygonal shape.
In particular for making self-piercing rivets and self-piercing nuts the volume of the blank should be greater than the calculated volume of the finished fastener, with the excess volume being dependent on the diamter and the length of the blank. The excess volume should be about between 0.5% and 2% of the volume of the finished fastener. The excess volume ensures that the length of the fastener can be maintained substantially constant, while the excess volume of the blank may result in a slight excess volume of the finished fastener, for example at the head of the fastener where no precise dimensions are required. This allows to precisely design the self-pierc-ing ends of the shanks of self-piercing rivets so that they are particularly suited to perform the self-piercing operation in the setting process.